The present invention relates to a device for applying load to a rotating shaft. Particularly, the present invention relates to an improvement of a device for applying load to a rotating shaft not mechanically but magnetically.
A device of this invention can be used for example as a load applying device for various motors or as a load applying device for an ergometer and the like.
It is necessary for a load applying device to apply continually an arbitrary load (brake torque) to a rotating shaft of an apparatus which receives the load. Therefore an excellent durability is required for such a load applying device.
Such a load applying device in the prior art will be described in the following, by taking an example of a load applying device used in an ergometer.
The Japanese patent Publication Gazette No. 8267/1983 discloses an example of the prior art concerning a total structure of a training apparatus of the pedal type including a load applying device (a training apparatus of this type being referred to in this specification as an "ergometer"). As described in the above stated gazette, the load applying device of an ergometer is generally of the system in which the magnetic field of an exciting coil generates eddy current in an object such as an eddy current plate to which brake torque is to be applied (this system being referred to hereinafter as the "eddy current system"). This system is advantageous in that little friction is caused in the apparatus and control can be made relatively easily, as compared with a system in which brake torque is applied by mechanical friction force.
The Japanese Utility Model Laying-Open Gazette No. 166058/1981 discloses a brake device (a load applying device) of an ergometer where an exciting coil is provided on one of the side surfaces of a rotating disc to which brake torque is applied.
The Japanese Utility Model Publication Gazette No. 8509/1972 shows an example of a heat radiation structure in a load applying device of the eddy current system.
One of the important conditions required for a load applying device is that load to be applied to an object, namely, brake torque can be set to an arbitrary fixed value, irrespective of the rotation speed of the object.
One reason for this is that in the case of an ergometer, for example, calculation of the kinetic amount of a person exercising using the ergometer cannot be made correctly, as desired, if the brake torque of a load applying device changes dependent on the rotation speed of the pedals, thus making the ergometer perform its function unsatisfactorily.
Another reason is that the brake torque applied by a load applying device can be generally utilized for many purposes if it can be set freely to an arbitrary fixed value independently of the rotation speed of an object to which the torque is applied.
None of the above stated conventional load applying devices of the eddy current system satisfy the aforesaid condition, all of them involving a disadvantage that if the speed of stepping on the pedals, namely, the rotation speed of the crankshaft changes, the applied brake torque also changes irregularly.
In addition, the devices of the later two gazettes mentioned above involve another disadvantage that a structure for forming a magnetic path becomes complicated.
The present invention pertains to a device for applying load to a load shaft rotating at a speed higher than a prescribed rotating speed, comprising:
a rotating disc coupled to said load shaft so as to be rotated, and
an electromagnet disposed in a position where main surfaces of said rotating disc are inserted in said eletromagnet but are not in contact therewith, so that magnetic flux traversing the surfaces of said rotating disc is generated to apply brake torque to said rotating disc.
If the present invention is applied in an ergometer as a preferred embodiment, the invention further comprises:
a crankshaft having pedals, and
a speed increasing mechanism provided in association with said crankshaft and said load shaft for increasing the rotation speed of the crankshaft to transmit the increased rotation speed to the load shaft, a speed increase ratio at the time of transmission being selected so that the rotation speed of the load shaft may be higher than the above stated prescribed rotation speed with the rotation speed of the crankshaft being in the range of practically applicable speed.
Since a closed magnetic path is formed by the rotating disc and the electromagnet facing both sides of the rotating disc, eddy current is generated efficiently in the rotating disc.
Further, since the rotation speed of the load shaft is set to a value higher than a prescribed rotation speed, brake torque applied to the rotating disc by the eddy current generated in the disc functions in a saturated region. Therefore, stable brake torque is applied irrespective of the rotation speed of the load shaft.
In the case of a preferred embodiment of the invention adopted in an ergometer, the rotation speed of the load shaft is maintained at a value higher than the above stated prescribed speed by the speed increasing mechanism and accordingly, brake torque is stably applied.